1. Field of the Invention
This invention relates to integrated circuit packaging technology, and more particularly, to a method of fabricating a FCBGA (Flip-Chip Ball-Grid-Array) package with molded underfill.
2. Description of Related Art
BGA (Ball Grid Array) is an advanced type of integrated circuit packaging technology which is characterized by the use of a substrate whose front side is mounted with a semiconductor chip and whose back side is mounted with a grid array of solder balls. During SMT (Surface Mount Technology) process, the BGA package can be mechanically bonded and electrically coupled to a printed circuit board (PCB) by means of these solder balls.
FCBGA (Flip-Chip Ball-Grid-Array) is a more advanced type of BGA technology which is characterized by that the semiconductor chip is mounted in an upside-down (i.e., clip chip) manner over the substrate and bonded to the same by means of a plurality of solder bumps attached to the I/O pads thereon.
As the flip chip is readily bonded in position over the substrate, however, an undergap would be undesirably left between the flip chip and the substrate. If this flip-chip undergap is not underfilled, it would easily cause the flip chip to suffer from fatigue cracking and electrical failure due to thermal stress when the entire package structure is being subjected to high-temperature conditions. As a solution to this problem, it is an essential step in flip-chip package fabrication to fill up the flip-chip undergap with an electrically-insulative material into the flip-chip undergap. The underfilled material, when hardened, can serve as a mechanical reinforcement for the flip chip to cope against thermal stress.
Conventionally, there are many methods that can be used to perform the above-mentioned flip-chip underfill process. One method is the molded-underfill process, which can fill the flip-chip undergap incidentally through the molding process to fabricate the required encapsulation body (or called molded compound). One example of the molded-underfill process is depicted in the following with reference to FIGS. 1A-1D (note that FIGS. 1A-1D are simplified schematic diagrams showing only the parts related to the invention; the actual layout on the FCBGA package may be much more complex).
FIG. 1A is a schematic sectional diagram showing a semi-finished FCBGA package assembly before molding; and FIG. 1B shows a bottom view of the semi-finished FCBGA package assembly of FIG. 1A. As shown, the semi-finished FCBGA package assembly includes: (a) a substrate 110 having a front surface 110a and a back surface 110b; (b) a semiconductor chip 120 mounted in an upside-down (i.e., flip chip) manner by means of a plurality of solder bumps 121 over the front surface 110a of the substrate 110; (c) an array of solder-ball pads 130 formed over the back surface 110b of the substrate 110, which are used for subsequent attachment of an array of solder balls (not shown) thereon; and (d) a solder mask 140 which is predefined with a plurality of openings 141 to expose the solder-ball pads 130.
As the semiconductor chip 120 is readily mounted in position over the substrate 110, however, a flip-chip undergap 120a would be undesirably left between the semiconductor chip 120 and the substrate 110. If this flip-chip undergap 120a is riot underfilled, it would easily cause the semiconductor chip 120 to suffer from fatigue cracking and electrical failure due to thermal stress when the entire package structure is being subjected to high-temperature conditions. One solution to this problem is to perform a molded-underfill process.
To facilitate the molded-underfill process, it is required to drill a vent hole 111 through the substrate 110 and the solder mask 140 at the central point of the area where the semiconductor chip 120 is mounted. Conventionally, the vent hole 111 is formed by drilling through the vent hole 111 and the solder mask 140, so that the part of the vent hole 111 in the solder mask 140 is equal in inside diameter as the part in the substrate 110.
Referring further to FIG. 1C together with FIG. 1D, during the molding process, the semi-finished FCBGA package assembly is disposed in a molding tool (not shown), and an encapsulation material, such as epoxy resin, is injected into the molding tool (not shown) to form an encapsulation body 150 to encapsulate the semiconductor chip 120.
During the forgoing molding process, the encapsulation material will also infiltrate into the flip-chip undergap 120a. Owing to the provision of the vent hole 111, the air in the flip-chip undergap 120a can escape to the outside atmosphere, thus allowing the encapsulation material to infiltrate unresistingly into the entire flip-chip undergap 120a and thereby form a molded underfill layer 122 between the semiconductor chip 120 and the substrate 110.
One problem to the foregoing molded-underfill process, however, is that the encapsulation material would further infiltrate all the way through the vent hole 111 to the bottom side of the solder mask 140 (the marching path is indicated by the arrows in FIG. 1C), and thus flash over the exposed surface of the solder mask 140 and possibly over the nearby solder-ball pads 130. Since the encapsulation material is electrically-insulative, the mold flash over the solder-ball pads 130 would degrade the bonding between the solder-ball pads 130 and the subsequently attached solder balls (not shown) thereon.
Related patents, include, for example, the U.S. Pat. No. 6,038,136 entitled xe2x80x9cCHIP PACKAGE WITH MOLDED UNDERFILLxe2x80x9d. This patent discloses a FCBGA package that is underfilled through molded-underfill process. The utilization of this patent, however, still has the above-mentioned problem of mold flash.
It is therefore an objective of this invention to provide a method for fabricating a FCBGA package with molded underfill, which can help to prevent mold flash over exposed surface of the resulted package through the vent hole, so as to assure the quality of the outer appearance of the resulted package.
It is another objective of this invention to provide a method for fabricating a FCBGA package with molded underfill, which can help to prevent mold flash over exposed surface of the resulted package, so as to assure the quality of the bonding between the solder-ball pads and the solder balls attached thereon.
In accordance with the foregoing and other objectives, the invention proposes an improved method for fabricating a FCBGA package with molded underfill.
Broadly recited, the method of the invention comprises the following steps: (1) preparing a substrate having a front surface and a back surface; the substrate being formed with a vent hole at a predefined location; the vent hole having an exit in the back surface of the substrate, (2) forming a plurality of solder-ball pads over the back surface of the substrate; (3) forming a solder mask over the back surface of the substrate; wherein the solder mask is predefined with an array of pad openings to expose the solder-ball pads and a mold-buffering opening aligned to the vent hole; and wherein the mold-buffering opening is dimensioned to be greater in width than the inside diameter of the vent hole; (4) mounting a flip chip over the front surface of the substrate; wherein a flip-chip undergap is left between the flip chip and the substrate; and (5) performing a molding process to form an encapsulation body through the injection of a encapsulation material to encapsulate the flip chip; wherein the vent hole allows the air in the flip-chip undergap to escape to the outside atmosphere, thereby allowing the encapsulation material to infiltrate unresistingly into the flip-chip undergap and form a molded-underfill layer; and wherein as the encapsulation material flows to the exit of the vent hole, the encapsulation material is contained with the solder mask""s mold-buffering opening.
By the invention, there would substantially exist no mold flash over the exposed surface of the solder mask and the solder-ball pads; and therefore, the resulted FCBGA package would be assured in the quality of its outer appearance and the bonding effect between the solder-ball pads and the subsequently attached solder balls thereon.